Gamma encoding/decoding

[ivovanderlans]

I'd like to add gamma encoding/decoding which I need for my project. It's just about reading and writing the gAMA chunk, so no transformations are involved (which seems a responsibility for the crate user). I have already created a local branch off the master branch, have run the tests (passing), verified the decoded values against the png test-suite, but have no permission to publish the branch, nor create a pull-request.

[HeroicKatora]

nor create a pull-request.

Pull requests should be open for everyone. The way Github work you'll need to open them from your own fork of the repository :)

[ivovanderlans]

Ah, this is going to take a few days (this would be my first contribution to github-like systems)

[de-vri-es]

I believe your PR (#244) was merged, so this issue can be closed 🎉. Thank your for implementing it :D